A diffractive lens, which has a concentric diffraction grating portion on the surface of an aspheric lens, is known as a lens that would realize higher image capturing performance than an aspheric lens. By achieving not just the refraction effect of an aspheric lens but also diffraction effect, a diffractive lens can reduce significantly various kinds of aberrations such as chromatic aberration and field curvature. Particularly with a diffraction grating portion, of which the cross section is either blazed or consisting of fine steps that are inscribed to each other in a blazed pattern, the diffraction efficiency of a particular order with respect to a single wavelength can be increased to almost 100%.
Suppose a blazed diffraction grating portion 92 has been formed on the surface of a base 91 with a refractive index n(λ) as shown in FIG. 9. The diffraction step d of a diffraction grating portion, of which the mth-order diffraction efficiency (where m is an integer) becomes 100% theoretically with respect to a light ray 93 that has been incident thereon perpendicularly with a wavelength λ, is calculated by the following Equation (1):d=mλ/(n(λ)−1)   (1)where the refractive index n(λ) indicates that the refractive index is a function of wavelength.
As can be seen from this Equation (1), as the wavelength λ varies, the d value that makes the mth-order diffraction efficiency 100% also varies. Although the diffraction efficiency is supposed to be of the first-order (i.e., m=1) in the following example, m is not always one.
FIG. 10 shows the first-order diffraction efficiency of a light ray that has been incident perpendicularly onto a diffraction grating portion of polycarbonate that has a diffraction step of 0.93 μm. Since the diffraction step d of the diffraction grating portion has been determined by substituting a wavelength of 550 nm into Equation (1), the diffraction efficiency of the first-order diffracted ray becomes almost 100% at a wavelength of 550 nm. The first-order diffraction efficiency has wavelength dependence, and therefore, decreases to approximately 50% at a wavelength of 400 nm. Once the first-order diffraction efficiency has declined from 100%, unnecessary diffracted rays, including zero-order, second-order and minus-first-order ones, are produced.
However, if light falling within the entire visible radiation range (i.e., in the wavelength range of 400 nm through 700 nm) is made to be incident on an aspheric diffractive lens, on which a diffraction grating portion such as the one shown in FIG. 9 has been formed concentrically, the resultant color image will have a lot of noticeable flare. Such a flare is caused by unnecessary diffracted rays other than the first-order one that should be used for producing a subject image. Among other things, the bigger the difference in luminance between the subject and the background, the more noticeable the flare will be.
When such a flare is produced, the diffraction grating shown in FIG. 9 can be used to capture an image in only limited situations. Specifically, in that case, the diffraction grating can be used only when the luminance of a subject to shoot is not as high as that of the background or when the resolution does not have to be high, for instance. That is why it cannot be said that the conventional technique has fully developed the potential of a diffraction grating, of which the image capturing performance could be much higher than that of an aspheric lens, were it used more appropriately.
To produce a color image with little flare using such a diffractive lens, somebody proposed a technique for reducing the wavelength dependence of the diffraction efficiency of a particular order (see Patent Document No. 1, for example). FIG. 11 illustrates a diffractive optical element as disclosed in Patent Document No. 1, which teaches applying and bonding a protective coating 113 that covers a diffraction grating portion 112 on a base 111. In that case, the diffraction step d′ of the diffraction grating portion that makes 100% the first-order diffraction efficiency of a light ray striking the diffraction grating portion 112 perpendicularly (i.e., at an angle of incidence θ of zero degrees) is given by the following Equation (2):d′=mλ/|n1(λ)−n2(λ)|  (2)where λ is the wavelength, m is the order of diffraction, n1(λ) is the refractive index of the base material, and n2(λ) is the refractive index of the protective coating material. If the right side of Equation (2) becomes constant in a certain wavelength range, the mth-order diffraction efficiency no longer has wavelength dependence in that wavelength range. Such a condition is satisfied if the base and the protective coating are made of an appropriate combination of a high-refractive-index, high-Abbe-number material and a low-refractive-index, low-Abbe-number material. By making the base and the protective coating of such appropriate materials, the diffraction efficiency with respect to perpendicularly incident light can be 95% or more in the entire visible radiation range. It should be noted that in this configuration, the materials of the base and the protective coating could be changed with each other. Also, the height d′ of the diffraction step of the diffraction grating portion becomes greater than the height d of the diffraction step of the diffraction grating portion with no protective coating to be calculated by Equation (1).
The diffractive lens shown in FIG. 11 produces only a few unnecessary diffracted rays other than the first-order one, and therefore, will hardly cause a flare that is a problem with the diffractive lens shown in FIG. 9. As a result, a good image can be produced with high resolution.
As can be seen, it is very effective to form the diffraction grating portion shown in FIG. 11 on the surface of an aspheric lens in order to produce an image with high resolution. In the following description, a diffractive lens to be used mainly for image capturing purposes will be referred to herein as a “diffractive imaging lens”.